Individual components of an optical system typically need to be aligned in order for the system to work efficiently and a small misalignment of each component to each other can cause severe efficiency losses or deviation in the optical path. There are currently two main techniques to assemble multiple optical components together: “Passive” and “Active” alignment techniques.
Classical alignment of optical components includes the use of complex external mechanical structures to accurately hold each of the individual components in their position. This fixation system is also known as Passive alignment method. Passive alignment methods are widely used for imaging optics, where the position of optical components does not need to be adjusted after the initial fixation. Depending on the complexity, size and precision of the optical system, the fabrication of such mechanical holding structure can be too complex to be fabricated with conventional machinery technologies. Additionally, the fabrication yield can be drastically decreased, then increasing the associated cost related to the assembling of such an optical system.
For precise optical components alignment, it is most of the time required to use Active setup technology, where each component is placed one after the other and their position is adjusted while the system is running. Active alignment methods are widely used in the case of small components using coherent radiation, such as laser diodes and other semi-conducting optical elements.
However, Active alignment methods can become very time consuming, and cost generating if more components have to be aligned together in order to form a complete optical system.
U.S. Pat. No. 5,833,202 to Adreasch Wolfgang, describes a mechanical fastening system in which standard micro-optical elements are initially mounted on respective one-piece frames, which in turn may be secured in position on a mounting plate. This fastening system involves complex assembly steps and provides poor alignment capabilities.
WO 99/26754 to Remy de Graffenried, describes a method for fixing miniaturized components onto a base plate by mean of a soldering joint. The disclosed method is very expensive to operate and provides relatively poor results with respect to alignment precision.
The document U.S. Pat. No. 7,238,621 describes a method for fabricating an optical device and micromechanical device, wherein both devices are monolithically integrated with a substrate. The optical surfaces and micromechanical devices are each formed in an etch step that is well-suited for forming that device. The disclosed embodiments enable the optical surface and micromechanical device to be fabricated irrespective of severe topography on the surface of the substrate. The etching steps required in this method require specific expensive equipment and involve complex manufacturing processes.
U.S. Pat. No. 6,649,435 (US2002/0086458A1) discloses a system and method of aligning a micro-mirror array to the micro-mirror package and the micro-mirror package to a display system. The system and method improve the alignment of the micro-mirror array to the display system by using a consistent set of precision reference regions. The micro-mirror package substrate engages an alignment fixture portion of a die mounted during the die mount operation, and a similar fixture when installed in a display system. The package substrate is held by the predefined regions on two edges and the three predefined regions on the top surface. When mounting the device in the package optical techniques may be used for x-y plane alignment. The use of packages or sockets increases the system complexity and considerably reduces the precision.
WO2007/120831A2 describes an integrated photonic module in which a plurality of optical components are maintained and aligned with an alignment frame.
Generally speaking, all those documents involves methods and systems featuring complex steps, and do not provide precise and reliable alignment of all the components.
For micro-projection applications, high accuracy assembling and alignment methods have to be used; this is notably the case for micro-mirror based scanning projection systems.
Micro-Electro-Mechanical-Systems (MEMS) having resonating micro-mirrors, are currently being used for projection purposes. The projection can either be done using a single mirror moving along two central and perpendicular axis (two degrees of freedom 2DOF), or two mirrors moving along a central axis (one degree of freedom 1DOF) both placed at 90 degrees one to each other. FIGS. 1 and 2 illustrate the two projection schemes. In FIG. 1A, the image is created by centering a collimated laser beam produced by lased 101 on the middle of the 2DOF micro-mirror surface 102 within the frame 105. The laser beam is reflected and deviated in two directions, so as to project a scanned image on the projection surface 104. In FIG. 1B, the projected image is created by centering a collimated laser beam on the first 1DOF micro-mirror surface 102. The laser is reflected to a second 1DOF micro-mirror surface 103 with the rotation axis placed at 90 degrees compared to the first 1DOF micro-mirror. During the actuation of the mirror(s), the collimated laser beam can be pulsed at a specific frequency to create an image with bright, dark and grayscale parts. A monochromatic image is projected when a monochromatic laser source is used. A multi color image can be projected when multiple different monochromatic laser sources are used simultaneously.
According to the previous projection system design, a precise alignment is needed between the laser source and the micro-mirror. As an example, when the reflective and moving part of the micro-mirror has a size of 1 mm square and a Gaussian distribution laser beam with a diameter of 0.5 mm at is 1/e intensity has to be aligned to hit the center of the mirror, the maximum misalignment can be of the order of 50-100 μm.
One consequence of a misalignment in the optical systems is the loss of light in the optical path, which induces a darker projected image and it may increase the temperature inside the projection system. Another consequence of such a misalignment is the lower light uniformity in the projected image.
Another consequence of misalignment of MEMS micro-mirrors with optical components for projection application is the projected image distortion.
As seen in FIG. 2, the MEMS micro-mirror is a movable structure 202 that classically moves out of the handled substrate plane 201. Therefore, the micro-mirror cannot be in direct contact with optical components, such as a lens or prism, because this will obstruct the mechanical movement of the micro-mirror.
The alignment of optical components obtained according to the above known methods presents problems. In particular, for precise alignment of multiple optical components, machine placement resolution becomes a critical bottleneck and induces alignment errors.
A further problem with these optical assembly setups for projection systems using moving parts, MEMS parts for example, is the fact that small mechanical holders are used, and mechanical fabrication tolerance adds further alignment errors.
A further problem with the assembly of MEMS micro-mirrors with optical components is the fact that for small size or complex design, Active alignment cannot always be used and then large alignment errors can occur.
A further problem with the assembly of MEMS micro-mirrors with optical components where no Active alignment can be used, is the fact that alignment problems are not detectable during the optical assembly phase, but can be only detected when all laser and MEMS driving electronic components are assembled to the optical components, causing difficulty to repair the defected projection system.
A further problem with known assembly and alignment methods is the high cost.
Thus, there is a need for a novel assembly technique and design for micro-optical components with MEMS micro-mirrors and MEMS components in general, that do not present the above mentioned drawbacks, namely the problems of the assembly techniques and design obtained by known methods.
US2006/0274273A1 discloses an optical imaging engine comprising a polarizing beam splitter, a quarter-wave retarder, a MEMS imaging device, the latter two components having flat surfaces mutually cooperating with each other. However, most of the described components such as the light source and the beam splitter do not have a parallelepiped profile. Accurate alignment of all the system components is thus not possible or involves complex alignment methods.
US2008/0158519A1 discloses a method of aligning a micromirror array to the micromirror package and the micromirror package to a display system.